1. Field of the Invention
This invention relates to liquid crystalline compounds and mixtures.
2. Description
Liquid crystals have recently gained considerable importance as dielectrics in indicating devices, since the optical properties of such substances can be influenced by an applied voltage. Electro-optical devices based on liquid crystals are well known to the person skilled in the art and can be based on various effects.
The dielectric anisotropy .DELTA..epsilon. of a compound or mixture is the difference between the dielectric constant along the longitudinal molecular axis (.epsilon..sub..parallel.) and the dielectric constant perpendicular thereto (.epsilon..sub..perp.). In the case of non-liquid crystalline components the term "dielectric anisotropy" in the scope of the present invention signifies the extrapolated value (from liquid crystalline mixtures which contain this component) of the dielectric anisotropy at a temperature which lies 10.degree. C. below the extrapolated (virtual) clearing point.
Nematic and cholesteric liquid crystals with negative anisotropy of the dielectric constants (.DELTA..epsilon.=.epsilon..sub..parallel. -.epsilon..sub..perp. &lt;0) are orientated in an electric field with their longitudinal molecular axes perpendicular to the field direction. This effect is known and is used for the control of the optical transmissivity in various liquid crystal indicators, for example in liquid crystal cells of the light scattering type (dynamic scattering), of the so-called DAP type (deformation of aligned phases) or of the guest/host type (guest host interaction).
These "guest/host cells" comprise essentially a condenser, at least one electrode being transparent and the dielectric being formed from a nematic or cholesteric liquid crystal which contains one or more dichroic colouring substances. Since the usable colouring substances mainly have positive dichroism, i.e. the transition moment of the absorption of visible light lies approximately in the direction of the longitudinal molecular axis, the orientation of the liquid crystal with the molecular axes parallel to the surface of the plates generally corresponds to the coloured state and the homeotropic orientation (longitudinal molecular axes perpendicular to the surface of the plates) generally corresponds to the colourless condition of the cell. When a liquid crystal with positive dielectric anisotropy is used, its homogeneous orientation (which is achieved by treating the surface of the plates) is arranged homeotropic by the application of a voltage, i.e. the cell is switched from "coloured" to "colourless". In this manner colourless symbols are shown on a coloured background. On the other hand, when a liquid crystal with negative dielectric anisotropy is used, its homeotropic orientation (which is achieved by treating the surface of the plates) is arranged parallel to the electrode surfaces by the application of a voltage, whereby the reading of coloured image elements on a colourless background is made possible.
Further, for the improvement of the multiplex ratio in the multiplex control of liquid crystal indicators, especially of rotation cells and guest/host cells, there has been proposed a two-frequency matrix addressing (e.g. German Offenlegungsschriften 2,856,134 (Great Britain Pat. No. 2,013,014) and 2,907,940 (Great Britain Pat. No. 2,020,075)). In this case, use is made of the fact that the dielectric anisotropy of liquid crystals, which upon application of a low-frequency voltage have a positive anisotropy of the dielectric constants, is negative in the case of high frequencies. In order to maintain the capacitive loss at a low level, the "cross-over frequency f.sub.c (dielectric relaxation frequency at which .epsilon..sub..parallel. =.epsilon..sub..perp.) of such liquid crystals should be as low as possible and should not lie above about 20 kHz. Further, the total sum of the dielectric anisotropies should be as large as possible not only below but also above the cross-over frequency. It has, however, been found that the substances, which are especially suitable for the two-frequency addressing, at frequencies above the cross-over frequency generally have a smaller absolute dielectric anisotropy than below the cross-over frequency. This disadvantage can be eliminated by adding compounds with negative dielectric anisotropy and suitable relaxation behaviour.
A series of liquid crystalline compounds with weakly negative dielectric anisotropy is already known. On the other hand, still relatively few liquid crystal components with large negative anisotropy of the dielectric constants are known. Moreover, the latter generally have disadvantages such as, for example, poor solubility in mixtures, high viscosity, high melting points, strong smectic tendencies, chemical instability or large melting point depressions in mixtures. There accordingly exists a need for further compounds with negative dielectric anisotropy which allow the properties of mixtures for the widest variety of electro-optical applications to be improved.
The present invention provides novel liquid crystalline compounds and mixtures with such improved properties.